Solid-state imaging devices are generally classified into two categories according to the method by which a signal is read out: CCD devices, which use CCDs (charge coupled devices) to transfer signal charges and read a signal simultaneously from a plurality of pixels, and MOS devices, which use read-out circuits comprising MOS transistors, formed for each pixel, to read a signal by selecting one pixel after another.
In recent years, MOS solid-state imaging devices, especially CMOS solid-state imaging devices that are produced with a CMOS (complementary MOS) process, have received much attention as image input elements for portable imaging apparatus such as small PC cameras, because they can be driven with low voltage and low power consumption, and because they can be integrated on one chip together with peripheral circuits.
MOS solid-state imaging devices are classified into two categories according to the read-out circuit that is formed for each pixel: passive devices, which directly read the signal charge that accumulates in a photo-receiving portion into an output line, and active devices, which amplify the potential difference that occurs due to the accumulation of the signal charge with an amplifying circuit before giving it out. FIGS. 9 and 10 are cross-sectional views showing structures of pixels in conventional MOS solid-state imaging devices. FIG. 9 shows a pixel in an active solid-state imaging device. The signal charge is transferred from the photo-receiving portion to a detecting portion. The potential difference occurring at the detecting portion is given out. Each pixel comprises a photo-receiving portion and four transistors: a charge transfer transistor, an amplification transistor, a reset transistor and a select transistor. The charge transfer transistor is a MOS transistor including a photo-receiving portion 53, a detecting portion 54a and a lightly doped drain portion (hereinafter, referred to as "LDD portion") 54b, which are formed in a silicon substrate 50, an insulating film 51 formed on the silicon substrate 50, and a gate electrode 52 formed on the insulating film 51 between the photo-receiving portion 53 and the detecting portion 54a. The photo-receiving portion 53 corresponds to the source and the detecting portion 54a corresponds to the drain of the charge transfer transistor, respectively. FIG. 10 shows a pixel in an active solid-state imaging device giving out the potential difference occurring at the photo-receiving portion. Each pixel comprises a photo-receiving portion and three transistors: an amplification transistor, a reset transistor and a select transistor. The reset transistor is a MOS transistor including a photo-receiving portion 63, a charge drain portion 64a and a LDD portion 64b, which are formed in a silicon substrate 60, an insulating film 61 formed on the silicon substrate 60, and a gate electrode 62 formed on the insulating film 61 between the photo-receiving portion 63 and the charge drain portion 64a. The photo-receiving portion 63 corresponds to the source and the charge drain portion 64a corresponds to the drain of the reset transistor, respectively.
Not shown in the drawings, in the solid-state imaging devices shown in FIGS. 9 and 10, an interlayer insulating film made of silicon oxide film is formed on the photo-receiving portion. Furthermore, a light-blocking film made of metal having an aperture above the photo-receiving portion and a surface protection film are formed on the interlayer insulating film sequentially.
With increasing resolution of the solid-state imaging device and increasing miniaturization, the improvement of the sensitivity of the solid-state imaging device becomes an issue to be addressed. However, in conventional MOS solid-state imaging devices, an interlayer insulating film made of silicon oxide film is formed on the photo-receiving portion. According to the configuration, due to the difference of refractive index between the silicon substrate and the silicon oxide film, light reflects on the surface of the substrate and the amount of incident light that reaches the photo-receiving portion decreases. Accordingly, the sensitivity of the solid-state imaging devices decreases.